The processes for Water Influx Prevention, applied to oil or gas producing wells, make possible a lasting lowering in the fraction of water produced by the well and an increase in the production of oil or gas, if the overall production of the well can be maintained or increased. Polymers are sometimes employed but these remain limited to relatively low permeabilities of water-producing zones (on average less than 300 milliDarcy), because of the limitation in size of these polymers. As they are, in addition, mechanically, thermally and chemically unstable, they are rarely used alone and generally stabilizing and/or crosslinking agents are added to them. It is preferable to use gels formed of polymers, whether lost circulation gels or dilute gels (thus having a low concentration of polymer and of crosslinking agent). The polymer/crosslinking agent mixture is then injected into the well to be treated with delayed gelling kinetics, the gel only setting after a few hours in the formation around the well. However, these processes are regarded as not very reliable and often use polluting products based on chromium salts or resins. Furthermore, the processes based on gels do not make it possible to control either the gelling kinetics or the consistency of the gel; they engender a high risk of damage to the well, they bring about the retention and the adsorption of the crosslinking agent on the reservoir rock and they make it possible only with difficulty to place the gel in the water zones and to prevent the invasion of the oil or gas zones.
European patent applications EP 0 082 657 and EP 0 122 073 and U.S. Pat. No. 4,726,906 disclose the use of polyampholytes in the treatment of oil wells.
The increasing number of mature fields and the development of complex wells (horizontal wells, offshore wells, multibranched wells), combined with the lack of reliability of the techniques for the separation of bottom sediments, have rendered highly advantageous self-selective treatments for Water Influx Prevention, that is to say treatments which can be injected throughout the open interval of the well (bullhead injection), without using mechanical means for treating localized zones. In the case of such self-selective treatments, as the oil or gas beds are not protected during injection, it is essential to inject a formulation which has little effect on the permeability to oil or to gas. Water-soluble polymers or weak gels of water-soluble polymers, which are products referred to as RPMs (Relative Permeability Modifiers), greatly reduce the permeability to water of the rocks while having little effect on the permeability to oil or to gas. They act by forming an adsorbed layer of polymer at the wall of the pores, which remains swollen as water passes but contracts under the effect of the capillary forces as oil or gas passes. A good RPM product should thus be strongly and irreversibly adsorbed on the rock and should form a layer thickness well suited to the pore size, should have a good mechanical, chemical and thermal stability and should have a “soft” consistency which allows it to easily contract as oil or gas passes.
In the self-selective (bullhead) treatment, it is also essential for the placement of the product injected into the various beds to be optimized. The operator will seek to minimize the depth of penetration into the oil or gas beds, so as to preferentially invade the water beds. The placement can be controlled either by the use of diversion products which make it possible to temporarily protect the oil or gas zones, i.e. by virtue of a property inherent in the RPM agent, for example by choosing a product sufficiently large in size (a few microns) not to penetrate the oil beds (generally the least permeable).
When the processes of Water Influx Prevention are applied to water injection wells, this application, also known as “Profile Control Treatment”, makes it possible, by injecting a product of polymer or polymer gel type into the drain holes of high permeability, to achieve better flushing of the reservoir and to prevent the water channeling phenomenon currently observed in heterogeneous reservoirs. As indicated above, microgels, due to their large size, will spontaneously invade preferentially the drain holes of highest permeability, in which the water moves, and will only to a very small extent spread into the zones of low permeability, which are the most saturated in oil. For this reason, they exhibit superior placement properties to those of the polymers.
As regards Sand Influx Prevention, the oil and gas industry sometimes uses resin-based formulations. The resin-curing agent mixture is injected diluted in a hydrocarbon solvent into the zone to be treated around the well. The setting time is sufficiently delayed to allow injection of the products under low pressure. The well is subsequently closed in order to allow the resin to cure and to consolidate the rock. Once cured, the resin behaves as a solid. It is therefore essential, if it is desired to keep the fluids flowing in the treated zone, either to add, to the formulation, a chemical agent which makes it possible to form channels in the resin or else to inject an inert gas during curing. However, the resin-based processes for Sand Influx Prevention are not used to any great extent as they employ organic compounds which are often not very safe and toxic; their setting kinetics are difficult to control. The interval treated cannot exceed a few feet in thickness and one to two feet in depth and the risks of damage to the well are high.
Enhanced Hydrocarbon Recovery by injection of polymers consists in thickening the injected water by addition of low concentrations of hydrophilic polymers, so as to provide better control of the mobility of the displacing fluid (water) with respect to the displaced fluid (hydrocarbon). The polymer most widely used for this application is an acrylamide/acrylate copolymer which is linear and of high molecular weight, the viscosifying power of which is very high and which is not adsorbed to any great extent on the reservoir rock. However, this type of polymer is not very stable above 70° C., is not very stable to shearing and is highly sensitive to salts, which bring about a significant reduction in its viscosifying power.